Abstract: In order to improve theoretical basis of deepwater drilling,design theory and method of gas injection rate during dual gradient drilling with riser gaslift were studied.Based on the multiphase flow characteristics during dual gradient drilling,a set of corresponding multiphase flow equations were established;in conjunction with definite conditions,a calculation model for gas injection rate during dual-gradient drilling process was developed.The model can be used to design the gas injection rate and analyze affecting factors and regularity in dual gradient drilling.Analysis shows that gas injection volume changes linearly with drilling fluid density,flow rate,water depth and wellhead back pressure,and is most sensitive to wellhead back pressure.It is rational to study gas injection rate with multiphase flow theory,which provides theoretical basis for dual gradient drilling operation.